1. Field of the Invention
The present invention relates to a device for measuring a size of a light spot, and particularly to a device for measuring a size of a spot of a recording laser beam to be focused on an optical recording disk so as to make an information carrying disk such as a video disk, a compact disk etc. in an optical recording apparatus.
2. Description of the prior art
The width of each of the so-called pits formed on the information carrying disk should be less than approximately 0.5 .mu.m. The diameter of the spot of the recording laser beam for forming pits on the optical recording disk is therefore adjusted to a length less than 0.5 .mu.m in the optical recording apparatus. The size of the spot of the recording laser beam is determined by a wavelength .lambda. of the recording laser beam and a numerical aperture NA of the objective lens which focuses the laser beam on the optical recording disk in the optical focusing system of the optical recording apparatus. It is however usually difficult to accurately realize a required size of the spot of the laser beam, because the wavelength .lambda. fluctuates and the numerical aperture NA and the aberrations of the lens in the optical focusing system distribute within the respective allowable ranges. The spot of laser beam actually appearing on the disk is therefore once measured by the beam spot size measuring device. Then, the optical focusing system of the optical recording apparatus is appreciated in view of the result of the measurement, and the optical focusing system is adjusted again as a whole so as to arrange the size of the spot of the laser beam at the required size.
A conventional light spot size measuring device comprises an optical magnifying system for magnifying in dimension an image of the light spot formed by the optical focusing system, and a video monitoring device including a camera tube receiving the magnified image on the pickup surface thereof, so that the size of the light spot can be measured on the basis of the output signal of the camera tube.
However, it has been still difficult to adjust the size of the beam spot within the required largeness by using such a conventional measuring device, because the output level of the camera tube is saturated due to the high intensity of the laser beam at the optical recording process.